The present invention relates generally to CATV network monitoring systems, and more particularly to methods and apparatus for pinpointing sources of common path distortion (CPD) or passive intermodulation distortion (PIM) in a two-way broad band hybrid fiber-coax (HFC) network.
One form of distortion in a modern two-way HFC CATV network is common path distortion (CPD). This distortion is the result of intermodulation of forward path (or down stream) signals at components in the common path like connectors, taps, terminations, etc. due to the “diode effect” caused by corrosion (see article “Common Path Distortions Explained, Bharat Patel, www.scte.org/chapters/newenland/-reference/cpd/cpd2htm). CPD produces composite triple beat (CTB) and composite second order (CSO) distortion products in both the return and forward paths of the HFC network. Additionally, the locations of CPD sources are often locations where noise is introduced into the CATV network. As shown in a presentation entitled, “RF Impairments in the Return Path & Their Impact on a DOCSIS Performance or the RF World according to Jack,” Jack Moran, Motorola, dated Sep. 23, 2003, CPD influence is one of the most important problems in return path service. In the forward path, CPD distortion products from the analog channels (e.g., in the range 50-550 MHz) produce degradation of the digital channels (e.g., in the range 550-860 MHz).
In view of the above, the problem of eliminating CPD is extremely important for modern HFC CATV networks. The challenge is to locate CPD sources among hundreds and sometimes thousands of nodes and miles of network cable.
There is a known system for detecting CPD sources in an HFC CATV network, described in International Application No. PCT/AU00/00235 from Mar. 24, 1999, International Publication No. WO00/57571, published on Sep. 28, 2000, by Rodney Eastment. This application discloses a method and apparatus for determining the time delay of a probing signal propagating between a CPD source and a headend station, and for determining the location of the CPD source based on the time delay and with the use of a cable network map and schematic diagrams (“Eastment System”).
A drawback of the Eastment System is that the probing signal has double sidebands separated by 59.5 MHz in the forward path (down stream) spectrum. In addition, the type of probing signal—a double sideband chirp pulse signal—requires that the sidebands be located in unoccupied portions of the forward path to avoid interference with the TV program signals. Thus, the probing signal in the Eastment System requires two unoccupied TV channels in the forward path spectrum. In a modern HFC CATV system, channel spectrum is very limited and valuable. It is a potential revenue source. If the Eastment System were employed to monitor continuously CPD in the network, two useful channels in the forward path would be lost, assuming that they are even available.
A second drawback of the Eastment System is that it relies solely on a double sideband probing signal. As a result, the bandwidth of the CPD signal generated in response to the probing signal (i.e., “echo signal”) will be limited. In Eastment, the echo signal is a second order intermodulation (difference) product of the probing signal, received at the headend in the return path spectrum (e.g., 5-50 MHz). The bandwidth of this signal is proportional to the resolution of the range that can be determined from this signal. That is, a wider bandwidth translates to a higher resolution. The bandwidth of Eastment's echo signal is 5 MHz. This bandwidth provides a target resolution of about 100 feet. In many instances, this resolution is not sufficient, considering that there could be many network devices (each being a potential CPD source) located within this span. Improved resolution is desired in order to reduce or eliminate ambiguity as to the identity of the source device.
An attempt to increase the bandwidth of the echo signal, to improve resolution, will be necessarily constrained by the resulting increase in bandwidth required for the corresponding probing signal in the forward path spectrum. This is especially so if the goal is to place the probing signal in the roll-off region of the forward path spectrum, to avoid interference. Thus, a system that relies solely on a double sideband chirp signal will have limited target resolution.
Another constraint on the bandwidth of the Eastment echo signal is the requirement that the signal be place in an unoccupied portion of the return path spectrum. This is a requirement because the presence of typical return path service signals, such as Internet and telephony, create an environment where it is difficult to distinguish the echo signal from the actual network traffic, or at least more so than in an unoccupied spectrum. Like the forward path, the return path spectrum has become crowded with the advent of Internet and telephone service. Thus, return bandwidth is valuable. Furthermore, in most cases, the only unoccupied portion of the return spectrum is at the lower frequencies, i.e., in the 5-15 MHz region. However, in this region, ingress noise and pulse noise are most common, making it difficult here as well to reliably detect and process the echo signal.
Another drawback of the Eastment System is that it relies solely on the second order difference product of a probing signal to detect CPD. In the Patel article, “Common Path Distortions Explained, it is suggested that the third order intermodulation products (caused by CPD) may be important for detection of CPD. In fact, the Patel article suggests that, in some cases, the third order products are more dominant than the second order products and, in other cases, are the only distortion products appearing from a CPD source. Further, it is known that the voltage-current response of a CPD source—a metal/oxide/metal junction due to oxidation (“diode effect”)—can be mathematically described with a polynomial of the third order. This analysis suggests that the third order products are likely to be more dominant than the second order products. Thus, a system that relies on both second and third order products is likely to detect CPD more reliably, especially in a typically noisy return path environment.
As pointed out in the Patel article, the appearance of CPD in the network may vary from night to day (probably due to temperature variations), may vary due to temperature variations in general, or may vary as a result of other factors. Generally, the Eastment System has been employed to respond to a CPD problem or to perform routine maintenance. Such efforts have overlooked the need to monitor the CPD environment over a longer period of time to ensure that all of the CPD sources are detected. Ideally, the network should be constantly monitored. With a system such as the Eastment System, however, this requirement would impose a substantial demand on technical personnel and other resources, and thus would be a costly proposition.
The coaxial cable portion of an HFC network has generally followed a tree-and-branch architecture. If a CPD source is located in a particular branch among multiple parallel branches, it may be difficult to resolve which branch it is located based solely on a range determination (“range ambiguity”). It is suggested in the Eastment application that such CPD sources can be located if the ranging resolution is high enough to determine whether the range would put the source at mid-span (between utility poles). If so, then that particular branch would be eliminated from consideration, because it is assumed that CPD sources do not usually occur at mid-span. This approach may be theoretically sound; however, it is dependent not only on resolution, but also on the accuracy of cable plant maps and/or schematics. Such maps or schematics are generally not accurate. The distance between cable plant devices and cable lengths are usually specified with a certain error. In many cases, the actual lengths of cable are not specified and only the distance between utility poles may be given. Further, the maps may not specify actual velocities of propagation of the cables in the network, and the maps may not reflect changes to the network over time. Thus, even with a very high resolution system, this approach may not succeed in practice, without accurate calibration.
In view of the short comings of standard cable plant maps, it is a necessity to go into the field and calibrate the network. This generally involves the transmission of a calibration signal from a calibration point in the network to the headend. There is a possibility that the calibration signal will be distorted or masked by an echo signal from a real CPD source located near the point of calibration. The Eastment application attempts to address this problem by proposing to shift the calibration signal in “phase time” from the actual CPD echo signal, to separate the two signals. This approach does not cancel out the CPD echo signal. Thus, the potential remains for interference between the CPD echo signal and the calibration signal. Such interference may lead to errors in calibration and, accordingly, errors in subsequent CPD range measurements.
Calibration of the network has generally required the use of technicians at two locations—the headend and the point of calibration. This process is time-consuming because it requires the coordination and communication of technicians at both sites. There is a greater chance of human error with multiple technicians performing the calibration than with one. In addition, there is the expense of employing multiple technicians for the calibration. Moreover, this approach ties up technicians that could be assigned to other maintenance tasks. Further, if other technicians are deployed to work in parallel on the CPD problem, they are left waiting for access to the system while a time-consuming calibration is performed.